Shell Scrap

if

• syntax
  

  # First form
  
  if condition ; then
      commands
  fi
  
  # Second form
  
  if condition ; then
      commands
  else
      commands
  fi
  
  # Third form
  
  if condition ; then
      commands
  elif condition ; then
      commands
  fi
  

• Example
  

  if [ -f .bash_profile ]; then
      echo "You have a .bash_profile. Things are fine."
  else
      echo "Yikes! You have no .bash_profile!"
  fi
  

• Condition
  

  Expression Description

  • -d file True if file is a directory.
  • -e file True if file exists.
  • -f file True if file exists and is a regular file.
  • -L file True if file is a symbolic link.
  • -r file True if file is a file readable by you.
  • -w file True if file is a file writable by you.
  • -x file True if file is a file executable by you.
  • file1 -nt file2 True if file1 is newer than (according to modification time) file2
  • file1 -ot file2 True if file1 is older than file2
  • -z string True if string is empty.
  • -n string True if string is not empty.
  • string1 = string2 True if string1 equals string2.
  • string1 != string2 True if string1 does not equal string2.
  • -a (AND) and -o (OR). The -a and -o operators are similar to the && and || operators used with exit statuses.

case

• syntax
  

  case word in
      patterns ) statements ;;
  esac
  

• example
  

  #!/bin/bash
  
  echo -n "Enter a number between 1 and 3 inclusive > "
  read character
  case $character in
      1 ) echo "You entered one."
          ;;
      2 ) echo "You entered two."
          ;;
      3 ) echo "You entered three."
          ;;
      * ) echo "You did not enter a number"
          echo "between 1 and 3."
  esac
  

Loops

• while
  
  • example
    

    #!/bin/bash
    
    number=0
    while [ $number -lt 10 ]; do
        echo "Number = $number"
        number=$((number + 1))
    done
    

• until
  

  The until command works exactly the same way, except the block of code is repeated as long as the condition is false.

  • Example
    

    #!/bin/bash
    
    number=0
    until [ $number -ge 10 ]; do
        echo "Number = $number"
        number=$((number + 1))
    done
    

• for
  
  • syntax
    

    for variable in words; do
        statements
    done
    

read

• example
  

  #!/bin/bash 
  
  echo -n "Enter some text > "
  read text
  echo "You entered: $text"
  

• option -t (time)
  

  The -t option followed by a number of seconds provides an automatic timeout for the read command.

  #!/bin/bash
  echo -n "Hurry up and type something! > "
  if read -t 3 response; then
      echo "Great, you made it in time!"
  else
      echo "Sorry, you are too slow!"
  fi
  

• option -s
  

  The -s option causes the user's typing not to be displayed.

$#

shift

Exit status

• Checking the exit status
  

  First, you can examine the contents of the $? environment variable. $? will contain the exit status of the last command executed.

  [me] $ true; echo $?
  0
  [me] $ false; echo $?
  1
  

example

TEMP_FILE=$TEMP_DIR/printfile.$$.$RANDOM

Watching your script run

• Method1
  

  #!/bin/bash -x
  

• Method2
  

  you can use the set command within your script to turn tracing on and off. Use set -x to turn tracing on and set +x to turn tracing off. For example.:

  #!/bin/bash
  
  number=1
  
  set -x
  if [ $number = "1" ]; then
      echo "Number equals 1"
  else
      echo "Number does not equal 1"
  fi
  set +x
  

trap command

trap 'command' signal

EXIT  从一个函数中退出或整个脚本执行完毕 
ERR   当一条命令返回非零状态时(代表命令执行不成功) 
DEBUG 脚本中每一条命令执行之前

• example
  
  • ERR

  ERRTRAP()
       {
         echo "[LINE:$1] Error: Command or function exited with status $?"
       }
       foo()
       {
         return 1;
       }
       trap 'ERRTRAP $LINENO' ERR
      abc
      foo
  

  • DEBUG

  #!/bin/bash
  trap 'echo “before execute line:$LINENO, a=$a,b=$b,c=$c”' DEBUG
  a=1
  if [ "$a" -eq 1 ]
  then
      b=2
  else
      b=1
  fi
  c=3
  echo "end"
  

tee command

ipaddr=`/sbin/ifconfig | grep 'inet addr:' | grep -v '127.0.0.1'
| tee temp.txt | cut -d : -f3 | awk '{print $1}'`
echo $ipaddr

$ cat temp.txt
inet addr:192.168.0.1  Bcast:192.168.0.255  Mask:255.255.255.0

use debug hook

if [ “$DEBUG” = “true” ]; then
echo “debugging”  #此处可以输出调试信息
fi

DEBUG()
{
    if [ "$DEBUG" = "true" ]; then
        $@　　
    fi
}
a=1
DEBUG echo "a=$a"

command

• 对"-x"选项的增强
  

  几个shell内置的环境变量：

  • $LINENO 代表shell脚本的当前行号，类似于C语言中的内置宏__LINE__
  • $FUNCNAME 函数的名字，类似于C语言中的内置宏__func__,但宏__func__{只能代表当前所} 在的函数名，而$FUNCNAME的功能更强大，它是一个数组变量，其中包含了整个调用链上所有的函数的名字，故变量${FUNCNAME¹}代表shell脚本当前正在执行的函数的名字，而变量${FUNCNAME²}则代表调用函数${FUNCNAME¹} 的函数的名字，余者可以依此类推。
  • $PS4 $PS4的值将被显示在“-x”选项输出的每一条命令的前面

  $ export PS4='+{$LINENO:${FUNCNAME[0]}} '
  

Simple usage

$PARAMETER
${PARAMETER}

Indirection

Case modification

${PARAMETER^} #modifies the first character to uppercase,
${PARAMETER^^} # all characters are converted
${PARAMETER,} #  , operator to lowercase
${PARAMETER,,}
${PARAMETER~} #~ reverses the case of first letter of words in the variabl
${PARAMETER~~}

Variable name expansion

${!PREFIX*} #This expands to a list of all set variable names beginning with the string PREFIX. 
${!PREFIX@}

Substring removal

${PARAMETER#PATTERN}  #The operator "#" will try to remove the shortest text matching the pattern, while "##" tries to do it with the longest text matching. 
${PARAMETER##PATTERN}
${PARAMETER%PATTERN} #except that Bash now tries to match the pattern from the end of the string
${PARAMETER%%PATTERN}

Search and replace

${PARAMETER/PATTERN/STRING}
${PARAMETER//PATTERN/STRING}
${PARAMETER/PATTERN} #removed
${PARAMETER//PATTERN}

Anchoring Additionally you can "anchor" an expression: A # (hashmark) will indicate that your expression is matched against the beginning portion of the string, a % (percent-sign) will do it for the end portion.

MYSTRING=xxxxxxxxxx
echo ${MYSTRING/#x/y}  # RESULT: yxxxxxxxxx
echo ${MYSTRING/%x/y}  # RESULT: xxxxxxxxxy

String length

${#PARAMETER}

Substring expansion

${PARAMETER:OFFSET}
${PARAMETER:OFFSET:LENGTH}

Use a default value

${PARAMETER:-WORD}

${PARAMETER-WORD}

Assign a default value

${PARAMETER:=WORD}

${PARAMETER=WORD}

Use an alternate value

${PARAMETER:+WORD}

${PARAMETER+WORD}
echo "The Java application was installed and can be started.${JAVAPATH:+ NOTE: JAVAPATH seems to be set}"

Display error if null or unset

${PARAMETER:?WORD}

${PARAMETER?WORD}

Arithmetic and command expansion

#Arithmetic expansion
$(( EXPRESSION ))
$[ EXPRESSION ]  #Please do not use the second form $[ … ]! It's deprecated. 
#Command substitution
$( COMMAND )
` COMMAND `

Brace expansion

{string1,string2,...,stringN}
{<START>..<END>}
<PREAMBLE>{........}
{........}<POSTSCRIPT>
<PREAMBLE>{........}<POSTSCRIPT>

aa

loop through the positional parameters

numargs=$#
for ((i=1 ; i <= numargs ; i++))
do
    echo "$1"
    shift
done
#or
for arg
do
    echo "$arg"
done
#while but it has the disadvantage to stop when $1 is empty (null-string)
while [ "$1" ]
do
    echo "$1"
    shift
done
# run as long as $1 is defined
while [ "${1+defined}" ]; do
  echo "$1"
  shift
done

mass usage

$*      $1 $2 $3 … ${N}
$@      $1 $2 $3 … ${N}
"$*"    "$1c$2c$3c…c${N}" # where 'c' is the first character of IFS.
"$@"    "$1" "$2" "$3" … "${N}"

Range Of Positional Parameters

${@:START:COUNT}
${*:START:COUNT}
"${@:START:COUNT}"
"${*:START:COUNT}"

touch all folders in a directory

find . -maxdepth 1 -mindepth 1 -type d -exec touch {} \+

touch all files in a directory

find * -exec touch {} \;
# set time to 29th of Feb 2012.
find * -exec touch -t 201202291000 {} \;